1. Field of the Invention
The present invention relates to an image display apparatus and, in particular, it relates to an image display apparatus in which a display attribute of a specific area is modifiable and which has function to correct the position of the specific area.
2. Description of the Related Art
Conventionally, an image display apparatus such as a display using a cathode-ray tube (“CRT display” below) or a liquid crystal display is connected to an information processor such as a personal computer via an analog interface. The display attributes (contrast or brightness or the like) of the specific area of a picture on the display apparatus can be modified. In an example, if a DVD (Digital Versatile Disc) image reproduced by a personal computer is displayed in a window defined in the CRT display screen, the display attribute is modified in such a way that the brightness of the portion of the window in which the DVD image to be reproduced is increased.
As described above, in an arrangement in which the display attribute of the specific area of a picture on an image display component is modified on the image display apparatus side, a display control circuit which outputs an image signal and a synchronization signal and a communication circuit which outputs a specific position information to the image display apparatus are provided in a personal computer. These signals are transmitted to the image display apparatus via a USB or a DDC. On the other hand, the image display apparatus comprises a communication circuit which receives the position information of the specific area from the personal computer, a display attribute control signal generation unit which generates a display attribute control signal, based on the position information and the synchronization signal, a circuit which converts the display attribute using the display attribute control signal, and an image display apparatus (below “display”) such as a CRT display or a liquid crystal display which displays an image.
The display attribute control signal generation unit comprises a microcomputer, a PLL circuit, a register, and a display attribute control signal generation circuit. The microcomputer stores the position information of the specific area which is supplied from the communication circuit in the register. The microcomputer determines a resolution based on the timing of the synchronization signal supplied from the personal computer, and stores values of the horizontal cycle time, the vertical back porch, and the horizontal back porch of the resolution.
The PLL circuit generates a dot clock DC, which is necessary for generating the display attribute control signal, from the horizontal cycle time stored in the register and the horizontal synchronization signal sent from the personal computer. The display attribute control signal generation circuit generates the display attribute control signal from the dot clock DC generated in the PLL circuit, the horizontal and vertical synchronization signals sent from the personal computer, and the position information, vertical back porch and horizontal back porch stored in the register. The dot clock DC is generated by multiplying the frequency of the horizontal synchronization signal, in the PLL circuit, according to the horizontal cycle time of the resolution determined by the microcomputer.
A picture displayed on the display is determined by a plurality of horizontal synchronization signals and a vertical synchronization signal. Accordingly, when the display attribute of the specific area of the picture on the display is modified on the image display apparatus side, the microcomputer counts the horizontal synchronization signals for the vertical back porch in response to the vertical synchronization signal, and therefore counts the horizontal synchronization signals to the start point of the specific area in the Y direction (vertical direction).
After counting the horizontal synchronization signal to the start point of the specific area in the Y direction (vertical direction), the dot clocks DC for the horizontal back porch are counted in the horizontal synchronization signal at the start point of the specific area in the Y direction to detect the start point of the specific area in the X direction (horizontal direction). After the dot clocks DC are counted to the start point of the X direction, the display attribute control signal is set ON. The display attribute control signal is set OFF when the dot clocks DC are counted to the end point in the X direction of the specific area. Likewise, the display attribute control signal is set on or off for each line, and this is repeated to the end point of the specific area in the Y direction. In the conventional image display apparatus, the display attribute of the specific area of a screen on the display is charged in the way mentioned above.
However, in the conventional method, the timing of the image signal input from the personal computer can deviate from a standard. For example, the horizontal cycle time may be incorrect, or the value of the vertical back porch or the horizontal back porch can be different. In such cases, an area of the picture whose display attribute is to be modified is not identical to an area of the picture whose display attribute is actually modified on the screen of the display.